Reduction in the noise produced by a rotor blade of a wind turbine

ABSTRACT

Rotor blades for wind power installations are known in many different forms. In a wind power installation the rotors or the rotor blades thereof represent the main source of sound. For reasons relating to acceptance and noise prevention laws, the aim should/must be that of minimizing the levels of sound emission as wind power installations are often also set up in the proximity of residential accommodation. The levels of sound emission which hitherto occur with a wind power installation or a wind power converter also mean that wind power installations are faced with resistance from populated areas because of the sound they produce and for that reason such installations can be accepted sometimes with difficulty or not at all as authorities responsible for planning permission refuse permission for wind power installations because of the existing environmental requirements, noise also being an environmentally polluting factor. Therefore the object of the invention is further to improve the noise emission of wind power installations. Wind power installation rotor blade comprising means for reducing the sound produced by a rotor blade, wherein the means is formed by a fluid-repellent layer and/or surface which is provided at least on a surface portion of the rotor blade.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Rotor blades for wind power installations are known in many differentforms. In a wind power installation the rotors or the rotor bladesthereof represent the main source of sound. For reasons relating toacceptance and noise prevention laws, the aim should/must be that ofminimizing the levels of sound emission as wind power installations areoften also set up in the proximity of residential accommodation. Thelevels of sound emission which hitherto occur with a wind powerinstallation or a wind power converter also mean that wind powerinstallations are faced with resistance from populated areas because ofthe sound they produce and for that reason such installations can beaccepted sometimes with difficulty or not at all as authoritiesresponsible for planning permission refuse permission for wind powerinstallations because of the existing environmental requirements, noisealso being an environmentally polluting factor.

2. Description of the Related Art

Many proposals have already been put forward for structurally modifyinga rotor blade of a wind power installation in such a way as to afford areduction in noise. By way of example reference is made here to thedocumentation as is disclosed in EP-A-0 652 367 or DE 196 14 420.5.

However a reduction in noise by virtue of structural measures on therotor blade is possible only to a limited extent.

BRIEF SUMMARY OF THE INVENTION

Therefore the object of the invention is further to improve the noiseemission of wind power installations.

In accordance with the invention that object is attained by a rotorblade having the features of claim 1. Advantageous developments are setforth in the further claims.

The invention is based on the realization that, if the surface of arotor blade is at least partially provided with a fluid- and/orice-repellent layer, the rotor blade also becomes rougher. Therefore,instead of providing the rotor blade with a coating comprising a coat ofpaint which imparts to the rotor blade on the top thereof a maximumdegree of smoothness, it is precisely the opposite that is done, namelyit is given a surface which is rough in respect of microstructure. Suchsurfaces are also known for example from lacquers or coatings whichperform the functionality of the so-called “lotus effect”, so thatwater/ice adheres only weakly to the surface. In that respect thecoating which is produced from a coat of paint comprises a kind of bedof nails of nano size. Those nano-nails of the bed not only roughen upthe surface of the rotor blade but also impart a lower level of hardnessto the surface because the individual nano-nails are also deformable intheir longitudinal direction or are considerably softer in respect oftheir structure, than the glass fiber coating of a rotor blade.

Thus the “lotus” coating on the rotor blade provides that the eddieswhich are formed on the top side of the rotor blade are restrained orchecked by the soft structure of the surface or energy is taken from theeddies of air so that overall—as has been noted—the sound which isproduced upon rotation of the rotor blade is reduced.

The micro-silicone paint “Lotusan” (trade mark of ispo GmbH, a companyof the Dyckerhoff Group) may be mentioned as a self-cleaning coating orpaint with which a considerable reduction in noise of a rotor blade canbe achieved in operation. That micro-silicone paint is marketed by thecompany under the article designation No 1950 and is described as beingdirt- and water-repellent. It is also possible for the coating to beformed by a sheet or foil, the surface structure of which forms awater-repellent layer. Self-cleaning surfaces (and the productionthereof) are also known from EP 0 772 514.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a wind power installation including a rotor.

FIG. 2 is a cross-sectional view of an enlarged portion of a surface ofa rotor blade from FIG. 1.

FIG. 3 shows a rotor blade 10 having shark skin pattern thereonaccording to principles of the present invention.

FIG. 4 shows a cross-sectional view of the enlarged shark skin patternof FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a front view of a wind power installation comprising a rotorwhich carries three rotor blades 10. FIG. 1 shows a wind powerinstallation of type E-40 from Enercon.

FIG. 2 shows a view in cross-section of a part of a rotor blade. It canbe seen in this respect that disposed on the surface is a coating 1 or acovering which forms a bed of nails 2 comprising “nano-nails” 3. Thespacing A between the nano-nails is in the range of between about 2 and250 μm and the height H of the nano-nails is in the range of betweenabout 2 and 250 μm. The nano-nails comprise for example hydrophobicpolymers or durably hydrophobized materials. Particularly good resultsfor reducing the sound produced by the rotor blade are achieved if thenano-nails are of a height of between 5 and 60 μm and their mutualspacing is approximately in the range of between 5 and 110 μm.

The coating of the rotor blade with a micro-silicone paint (for example“Lotusan”) also has the consequence that water (H₂O) or other fluids donot remain clinging to the rotor blade surface. This therefore alsotakes away from the outset the basis for any icing on the blade.

Preferably the coating is not applied entirely to the rotor blade butonly in the last third (as viewed from the rotor) of the rotor blade andthere preferably in the region of the rotor blade tip or at the rotorblade trailing and leading edges.

Due to the formation of the nano-nails 3 the surface of the rotor bladeis of very great irregularity or roughness so that the mass attractionof water drops 4 (molecules) and the rotor blade surface is notsufficient for the water molecules to remain clinging thereto. Thenano-nails therefore so-to-speak hold the foreign water molecules at aspacing relative to the surface 6 of the rotor blade, whereby theattraction force between the water molecules and the surface isdrastically reduced.

At the same time the nano-nails 3 have so-to-speak the function of a“(sound) shock absorber” because eddies (not shown) which naturally formon the surface of the rotor blade and which are responsible for thegeneration of sound impinge on the nano-nails which in turn, by virtueof their relatively great mobility, compared to the rigid glass fiberstructure of the rotor blade, can absorb the energy of the eddies andthus take energy away from the air eddies so that the sound is reduced.

The coating can be formed by an applied coat of paint or by a foil orsheet secured by adhesive.

The above-mentioned coating can be applied not only to a rotor blade orparts thereof, but also to other parts of the wind power installation,for example to the tower 7 of the wind power installation and/or to thecasing 8. The casing 8—which is usually also referred to as the pod—isdisposed at the head end of the tower and regularly encloses thegenerator of the wind power installation or other parts of the windpower installation which are not to be exposed directly to theenvironmental influences. In that respect the coating may be providednot only externally on the tower or rotor blade and/or the casing, butalso on the inside. For that purpose, it is advantageous if dripchannels (not shown) are provided on the inside and/or outside, by wayof which for example the water which runs off the tower and/or thecasing can be caught, collected and carried away in a controlledfashion. Such channels preferably extend substantially perpendicularly(or slightly inclined with respect to thereto) relatively to thelongitudinal axis of the tower on the wall of the tower and the liquidwhich is caught is carried away by a down pipe connected thereto.

Alternatively or supplemental to the above-described structure thereduction in the generation of noise can also be achieved by the rotorblade having a special surface in the manner of a “shark skin”. Thatsurface can be created by adding a sheet or foil coating. Such a foil orsheet is sold for example by 3M under the type designation 3M 8691 DragReduction Tape (Riblet Tape). That foil or sheet was developed as acommission from the aviation industry, with the aim of achieving asaving in fuel for aircraft by virtue of that specific “shark skin”surface.

The structure of such a “shark skin foil” is known for example frompublications by Diftrich W. Bechert (Abteilung Turbolenzforschung desDeutschen Zentrums für Luft- and Raumfahrt (DLR)—Turbulence ResearchDivision of the German Aerospace Centre). The structure of a “shark skinfoil” (coating) is also described in detail inter alia in EP fl 846 617,DE-C-36 09 541 or DE-C-34 14 554. For the avoidance of repetition thecontent of all the above-indicated publications is also to be deemed tobe content of the present application. As the sound in the case ofaircraft is essentially determined by the engines, the sound produced bythe aircraft is not reduced, especially as the sound levels which aregenerated by virtue of the dynamic events on the aircraft (aerofoil) arebelow the listening threshold and therefore cannot be perceived.

A foil in accordance with the principle of the shark skin (under acorresponding surface) was developed by an engineering team headed byDr. Dietrich W. Bechert of the Abteilung Turbolenzforschung desDeutschen Zentrums für Luft- and Raumfahrt (DLR) (translation:Turbulence Research Division of the German Aerospace Centre) at theTechinical University of Berlin.

FIG. 3 shows the case of such a “shark skin” foil the surface of thefoil has fine channels formed between ribs 11 extending in the flowdirection on rotor blade 10. Those channels are not continuous but aredisposed on panels (scales) 12 which in turn are arranged in mutuallydisplaced relationship, as shown in FIG. 3. In the illustrated example a“scale” 12 has five channels between ribs 11 which are of differentlengths and which are oriented with their longitudinal directionperpendicular (or parallel) to the radius r of the rotor blade of a windpower installation.

As shown in FIG. 4, in this case the height H of the ribs 11 that formthe channels is about between 30 and 70% of the channel spacing s andthe ribs are preferably of a wedge-shaped configuration with a taperangle of between about 5 and 600.

The standardized lateral rib spacing of the shark skin foil surface inthis case in accordance with the formula s⁺=(s/ny)*{squareroot}(tau₀/rho) is between 12 and 22, wherein s is the lateral ribspacing, tau₀ is the wall tension of a smooth reference surface which isexposed to the same flow, rho is the density of the flow medium (air)and ny is the kinematic viscosity of the flow medium (air). In this casethe standardized rib spacing s⁺ is preferably adjusted to peripheralspeed (or angular speed) of a rotor blade of a wind power installationin operation at nominal rating. Preferably in that respect it isadjusted to the peripheral speed of the rotor blade tip or the rotorblade tip region (between about 5 and 25% of the rotor blade length).

The channel spacings in that case is between 0.001 and 0.15 mm.

It is also possible for surface structures with a differing channelspacing and/or scale spacing to be provided over the entire rotor bladesurface so that adjustment of the standardized channel spacing is alwaysto the respective peripheral speed of the rotor in nominal operation.

Preferably the lateral attachments of the ribs also have a radius ofcurvature of a maximum of 50%, preferably a maximum of 20%, of thelateral rib spacing s.

It is also advantageous if the surface of the shark skin foil, betweenthe ribs, has a radius of curvature of at least 200% of the lateral ribspacing. That is shown on an enlarged view in cross-section in FIG. 4.

Initial tests have shown that the sound emission of a rotor with rotorblades which have the above-described shark skin foil (and thus also thecorresponding surface as described) could be reduced by between about0.2 and 3 dB (depending on peripheral speed and wind conditions).

A measure as an alternative to or supplemental to the above-describedsound-reduction measures can also involve providing portions of a rotorblade, in particular the rotor blade leading edge, with an anti-erosionlacquer or paint. For example a solvent-bearing 2-component PUR lacquerwith Teflon-like surface properties can be provided as such ananti-erosion lacquer. Hitherto, anti-erosion foils or sheets have beenglued onto rotor blade leading edges in order to prevent erosion of therotor blade leading edge due to dirt particles/rain/hail etc. Gluing onthat foil is very complicated and troublesome and has to be carried outwith an extremely great amount of care in order to prevent it fromrapidly becoming detached in operation. In spite of the greatest amountof care being applied, it nonetheless repeatedly happens that theapplied foils come loose, which under some circumstances can also resultin an increase in the sound level in operation, but at any event causeshigh servicing costs as the detached or protruding foil pieces (foilcorners) have to be re-secured to the rotor blade again or fresh foilshave to be fitted.

A sliding or slippery sealant as is offered by Coelan under thedesignation VP 1970M, is suitable as an anti-erosion lacquer with whichit is possible to eliminate the problems of the known anti-erosion foil.That involves a solvent-bearing 2-component PUR lacquer havingteflon-like surface properties and the following characteristics: Solidscontent: Component A about 60% Component B about 5% Mixture about 32%Flash point: −22° C. Density: Component A 1.11 g/cm³ (20° C.) ComponentB 0.83 g/cm³ (20° C.) Viscosity: Component A about 80 s DIN 4 (23° C.Component B <10 s DIN 4 (23° C.) Processing time: about 16 h in a closedcontainer Skinning: about 30 min (20° C.; 50% relative air humidity)Non-tacky after: about 2 h (20° C.; 50% relative air humidity)Completely dry: about 96 h (20° C.; 50% relative air humidity) Pendulumhardness: 147 seconds (in accordance with König; DIN 53157) Quickweathering: withstood 2350 h UV-A with the Q-panel apparatus (QUV-test)withstood 2430 h UV-B with the Q-panel apparatus Mixture ratio:Component A 100 parts by weight Component B 100 parts by weight

That lacquer was developed for boat building, but the use thereof inrelation to rotor blades for reducing the generation of noise hashitherto never yet been proposed and is highly advantageous because itmakes it possible to replace the known anti-erosion foil and eliminatethe problems thereof.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, U.S. patent application Ser. No.09/857,925 filed on Aug. 27, 2001; and International Application No.PCT/EP99/09691 filed on Dec. 9, 1999, are incorporated herein byreference, in their entirety.

1. A rotor blade comprising: a surface; a fluid repellent structurecoated on at least a portion of said surface, said structure having aplurality of nano-nails providing a degree of unevenness andirregularity on said portion of the surface such that water drops do notadhere to the rotor blade surface and rotor blade sound generation inoperation of the wind power installation is lessened.
 2. A rotor bladeaccording to claim 1 wherein the fluid-repellent structure is coated onat least those parts of the rotor blade where the sound is generatedupon rotation of the rotor blade.
 3. A rotor blade according to claim 1wherein the nano-nails are spaced from each other in the range ofbetween 2 and 250 μm and the height of the nano-nails are in the rangeof between 2 and 250 μm.
 4. A rotor blade according to claim 1 whereinthe nano-nails are spaced from each other in the range of between 5 and110 μm and the height of the nano-nails are in the range of between 5and 60 μm.
 5. A rotor blade according to claim 1 wherein the nano nailscomprise hydrophobic polymers or durably hydrophobized materials whichcannot be detached by natural rain.
 6. A rotor blade according to claim1 wherein the nano-nails are deformable along their longitudinaldirections.
 7. A rotor blade according to claim 6 wherein the portion ofthe rotor blade surface coated with the fluid repellent structure issofter than the uncoated portion of the rotor blade surface.
 8. A rotorblade according to claim 1 wherein the fluid-repellent structure furthercomprising a shark-skin-like structure.
 9. A rotor blade according toclaim 8 wherein the shark-skin-like structure of the rotor bladecomprises a plurality of ribs over which a turbulent flow, having a mainflow direction, passes, said ribs being orientated in the main flowdirection and spaced laterally in relation to the main flow direction,and wherein, said ribs have a height between 30% and 70% of a lateralrib spacing of the ribs.
 10. A rotor blade according to claim 9 whereinthe standardized lateral rib spacings ⁺=(s/ny)*{square root}(tau₀/rho) is between 12 and 22, wherein s isthe lateral rib spacing, tau₀ is the wall thrust stress of a smoothreference surface which is exposed to the same flow, rho is the densityof the fluid and ny is the kinematic viscosity of the fluid.
 11. A rotorblade according to claim 10 wherein the ribs are of a wedge-shapedconfiguration.
 12. A rotor blade according to claim 11 wherein a taperangle of the wedge-shaped configuration is between 10 and 60°.
 13. Arotor blade according to claim 1 wherein portions of the rotor blade iscoated with an anti-erosion lacquer providing teflon-like surfaceproperties.
 14. A rotor blade according to claim 13 wherein said portionof the rotor blade is the rotor blade leading edge.
 15. A wind powerinstallation comprising a rotor blade according to claim
 1. 16. A windpower installation comprising a rotor blade according to claim
 8. 17. Awind power installation comprising a first member and a second member,the first member including a tower and the second member including acasing which encloses at least one generator of the wind powerinstallation, wherein at least one of the members is provided with awater-repellent structure which is applied at least to a surface portionof said member.